The use of sulfur compounds in feed streams to ethylene steamcrackers (both thermal and catalytic) has been known for many years to reduce coking rates and to reduce CO (carbon monoxide) production in the pyrolysis section of steamcracker furnaces. Others characterize the contribution of the sulfur compounds with improved reaction speed and product selectivity along with delaying or decreasing the frequency of de-coking of the reactors (lengthening the time between de-coking procedures). The sulfur sources are called sulfiding agents and are believed to decompose to form sulfidic surfaces on the insides of the reactors.
Sources of sulfur compounds have included 1) naturally occurring sulfur in feed (i.e. use of sour feed), 2) inexpensive odiferous sulfur compounds, which are also used for hydrotreater catalyst sulfiding. In particular the natural sulfur in good feedstocks has worked well with Co/Mo catalysts. As poorer quality feedstocks were used the sulfur content had higher thermal stability and required extremely high temperatures to activate the catalyst. Also as more sophisticated catalysts were made with greater hydrogenating and cracking power, they benefited from higher degrees of sulfurization than was possible with the feedstock. The preferred inexpensive odiferous sulfur compounds include DMDS (dimethyl disulfide) or DMS (dimethyl sulfide).
DMDS has a flash point of 15° C., a boiling point of 110° C., a strong odor and is a powerful solvent for many polymers. DMS has a flash point of <−18° C. and a boiling point of 36–39° C. Both compounds have strong odors, require grounded equipment, and require operators in contact or potential contact with the chemicals to have goggles, gloves, and an air supplied respirator.